Contemporary fluid dispense systems are well suited for dispensing precise amounts of fluid at precise positions on a substrate. A pump transports the fluid to a dispense tip, also referred to as a “pin” or “needle”, which is positioned over the substrate by a micropositioner, thereby providing patterns of fluid on the substrate as needed. As an example application, fluid delivery systems can be utilized for depositing precise volumes of adhesives, for example, glue, resin, or paste, during a circuit board assembly process, in the form of dots for high-speed applications, or in the form of lines for providing underfill or encapsulation.
Early dispensing pumps included a syringe with a dispense tip and a pressured air/vacuum source. Air pressure was applied to a plunger in the syringe, causing the plunger to engage a fluid in the syringe, thereby initiating a dispensing operation by forcing the fluid out of the dispense tip. To halt operation, a vacuum was drawn on the plunger. In this manner, dispensing operations were controlled by regulating the air pressure/vacuum applied to the syringe. While this embodiment was adequate for certain applications, as technology evolved to demanded higher dispensing accuracy, its application became somewhat limited.
Contemporary dispensing pumps improved capability by increasing control over the timing and volume of the dispensing operation. This was accomplished through the integration of the feed screw into the dispensing pump system. Such systems comprise a syringe, a feed tube, a dispense cartridge, and pump drive mechanism. The syringe contains fluid for dispensing, and has an opening at its distal end at which a feed tube is connected. The feed tube is a flexible, hollow tube for delivering the fluid to the cartridge. The cartridge is hollow and cylindrical and includes an inlet neck at which the opposite end of the feed tube is connected. The inlet neck directs the fluid into the hollow, central cartridge chamber.
A feed screw disposed longitudinally through the center of the cylindrical chamber transports the fluid in Archimedes principle fashion from the inlet to a dispensing needle attached to the chamber outlet. A continuously-running motor drives the feed screw via a rotary clutch, which is selectively, actuated to engage the feed screw and thereby effect dispensing. A bellows linkage between the motor and cartridge allows for flexibility in system alignment.
Pump systems can be characterized generally as “fixed-z” or “floating-z” (floating-z is also referred to as “compliant-z”). Fixed-z systems are adapted for applications that do not require contact between the dispense tip and the substrate during dispensing. In fixed-z applications, the dispense tip is positioned and suspended above the substrate by a predetermined distance, and the fluid is dropped onto the substrate from above. In floating-z applications, the tip is provided with a standoff, or “foot”, designed to contact the substrate as fluid is delivered by the pump through the tip. Such floating-z systems allow for tip travel, relative to the pump body, such that the entire weight of the pump does not bear down on the substrate.
Such conventional pump systems suffer from several limitations. The motor and rotary clutch mechanisms are bulky and heavy, and are therefore limited in application for modern dispensing applications requiring increasingly precise, efficient, and fast operation. The excessive weight limits use for those applications that require contact of the pump with the substrate, and limits system speed and accuracy, attributed to the high g-forces required for quick movement of the system. The mechanical clutch is difficult to control, and coasts to a stop when disengaged, resulting in deposit of excess fluid. Clutch coasting can be mitigated by a longitudinal spring mounted about the body of the feed screw and urged against the chamber end to offer rotational resistance. However, the spring adds to the length of the cartridge, and contributes to system complexity.
The inlet neck feeds directly into the side of the feed screw or “auger”. Consequently, as the auger collects material from the small and circular inlet port, high pressure is required for driving the material into the auger body, because the auger threads periodically pass in front of the feed opening, preventing material from entering. This leads to inconsistent material flow. Additionally, the inlet neck is commonly perpendicular to the auger screw, requiring the fluid to make a 90 degree turn upon entering the pump. This further limits material flow and can contribute to material “balling” and clogging.
Overnight storage of dispensed fluids often requires refrigeration of the fluid and cleaning of the system. The syringe is typically mounted directly to a mounting bracket on the pump body such that the output port of the syringe passes through an aperture on the mounting bracket. The feed tube is then coupled to the output port on the opposite face of the bracket. Since the tube and bracket are on opposite sides of the bracket, removal of the syringe from the pump body requires dismantling of the tube and syringe, which can contaminate fluid material positioned at the interface during disassembly. Further, since the syringe and cartridge can not be removed and stored together as a unit, disassembly and cleaning of the cartridge is required. Additionally, the inlet neck is narrow and therefore difficult to clean.
Dispense pumps are commonly mounted on a positioning platform, or gantry system, that positions the pump along the Cartesian x, y and z axes, relative to the substrate. A computer, or controller, performs various dispensing tasks using the positioning platform to control the pump position according to commands that are programmed by an operator. As explained above, pump/platform systems currently in use in the field employ the aforementioned brush motor or clutch-based pumps. Such pumps operate in response to a time-period-based signal from the controller, the duration of which dictates the length of time the motor is on (or, for a continuously-running motor system, the length of time the clutch is engaged), and therefore the amount of fluid that is dispensed. For example, the rising edge of the signal may initiate rotation of the brush motor (or engage the clutch), and the falling edge may turn off the motor (or disengage the clutch). While such pumps are adequate for operations requiring relatively large dispensing volumes, at smaller volumes the system resolution is relatively limited, since the timing signal is relatively inaccurate at shorter time periods, and since residual motion in the clutch or brush motor is difficult to predict. Assuming the platform/pump controller to be a computer-based system, the time-period-based signal may be subject to even further variability, since initiation of the signal may be delayed while other tasks are processed by the computer.
Conventional dispensing pumps are further limited in that following a dispensing operation, or in between dispensing operations, material can continue to flow, or drip, from the pump and dispense tip. This can lead to excessive dispensing of the fluid, for example in the form of greater dispensed fluid volume than desired, or the dripping of fluid at undesired locations on the substrate. This is especially problematic for dispensing of materials of relatively low viscosity, which tend to flow or drip more freely.
Others have attempted to address this problem, with limited success. For example, U.S. Pat. No. 5,819,983 proposes a pump embodiment having a auger screw that is axially moveable between a flow position, in which material is permitted to flow through the outlet, and a sealed position, in which material is prevented from flowing. A pneumatic system is used to drive the screw downward and upward between the flow position and the sealed position. This system is however mechanically complex, owing to the number of moving parts, and can cause eventual wear on the inlet of the dispensing needle, where the auger screw comes in contact with the needle when in a sealed position. In addition, the vertical position of the auger must be set, which can further complicate setup and maintenance of the system. Wear and improper settings can lead to inaccurate volume dispensing, and mechanical complexity can lead to jamming.